The instant invention relates generally to a coating hood for applying a protective coating to hollow glass containers.
The desirability of applying protective coatings to the exterior of hollow glass containers has long been recognized. Such coatings, which include tin, titanium, or metallic compounds, other heat decomposable or organometallic compounds, protect the glass containers from surface damage, such as abrasions and scratches, which result in a loss of tensile strength for the glass containers. The need for high-tensile strength in glass containers is particularly acute when containers are mass-produced, move rapidly in close proximity along high-speed conveyor lines and are subsequently filled with carbonated beverages, beer, wine, foodstuffs, and the like that produce gaseous pressure within the container.
Such protective coatings are usually applied as the glass containers emerge in a heated, fully-shaped condition from a glassware forming machine, that is, at the "hot end" of the system. The containers are transported away from the forming machine by a conveyor. Temperatures in excess of 400 degrees Centigrade exist at the surface of the glass containers, so that when a heat decomposable inorganic metallic, or organometallic, compound is applied thereto, the compound decomposes immediately and is converted to a metallic oxide coating.
One well-known and previously widely used technique for applying a protective coating to the ht glass containers calls for spraying the opposite sides of the containers as they travel on a conveyor, in single file, through two longitudinally spaced, oppositely positioned spray heads. Each spray head covers approximately 180 degrees of the circumference of the bottle, so that at least two spraying stations are required to coat the entire bottle. Receivers are positioned at the opposite side of the conveyor in alignment with the spray head. Pressurized air with the coating compound entrained therein is discharged from each spray head at a significant pressure, while the receivers are usually maintained at a low pressure. The net pressure differential increases the velocity, and thus the effectiveness, of the coating compound. Coating systems of this nature are disclosed in U.S. Pat. No. 3,516,811, granted June 23, 1980, to G. L. Gatchet et al., and U.S. Pat. No. 3,684,469, granted Aug. 14, 1972 to W. C. Goetzer et al.
Gatchet et al. recognized that the deposition of a metallic oxide coating on the finish of the glass container passing on a conveyor through the open-sided coating apparatus was undesirable, as noted in column 3, lines 21-57. To control the location of the metal oxide deposit, as well as the uniformity of the deposit, Gatchet et al. relied upon spray heads producing a (theoretically) laminar flow that would pass laterally across the width of the conveyor, as shown in FIG. 4 of the patent.
The above-described coating systems, however, are "open-sided" and are thus adversely influenced by ambient conditions in the factory where the glass containers are formed. The ambient conditions of prime concern are rapidly moving air currents, the moisture in the air, and the potentially toxic and/or corrosive fumes and pollutants issuing from the spray heads. Air currents can cause turbulent conditions at the spray heads that will cause a preferential or uneven, application of the protective coating, and some of the coating will accumulate on the bottle finish. The rapidly moving air currents disrupt the laminar flow patterns that are theoretically possible with "open-sided" systems, and the capability of uniformly, and consistently, applying the same thickness of coating is severely reduced. To compensate for air currents, the above-described systems are operated at higher pressures, and with greater throughput of coating compound, than would be required under quiescent conditions. The moisture in this hostile atmosphere causes hydrolysis loss by rendering some of the compound unfit for its intended purpose. Further, the escape of potentially toxic fumes into the atmosphere at the work place constitutes an occupational health hazard that may violate Federal, state and local ordinances. Also, the toxic fumes are highly corrosive and attack various components of the glass factory, such as the blowers, exhaust systems, conveyors, and even roofs, leading to increased plant maintenance costs. Furthermore, the efficiency of these "open-sided" systems is low, since much of the relatively expensive coating compound is wasted.
A second, well-known, and widely employed technique for applying a protective coating to hot glass containers relies upon a formed, sheet metal coating hood with spray heads and associated receivers situated therein. The coating hood obviates many of the problems associated with the open-ended spray systems discussed above. For example, the hood isolates the glass containers to be coated from the ambient conditions and furnishes a controlled atmosphere that enhances the coating operations. The hood includes an exhaust system which captures most the air entrained coating compound that does not adhere to the containers, thus reducing the problems of venting the system and minimizing the opportunity for the coating compound to attack building components. Also, such hood can significantly raise the coating efficiency of the systems, with attendant cost savings.
Representative coating hoods are disclosed in U.S. Pat. No. 3,819,404, granted June 25, 1974 to Addison B. Scholes and Joseph J. Kazlowski, in U.S. Pat. No. 3,933,457, granted Jan. 20, 1976 to Addison B. Scholes, in U.S. Pat. No. 4,389,324, granted June 21, 1983 to Georg H. Lindner and U.S. patent Ser. No. 684,046, to Georg H. Lindner et al. The most recent patent to Lindner et al. presents a coating hood including a tunnel for allowing containers to pass therethrough, and a vertically adjustable flat roof for accommodating containers of various sizes. At least two jet slots are located in each side wall, and at least two receivers or suction slots are aligned therewith. The jet slots and suction slots are interspersed opposite to each other in each side wall. The coating compound is introduced through at least one feedpoint, and blowers secured to the side wall furnish high velocity air containing the coating compound to the interior of the hood. Baffles are situated in the flow path of the high velocity air so that the jets issuing from the jet slots are well defined and thus better suited for their intended function.
In a unique fashion in the Lindner coating hood, two or more recirculating loops are defined by the judicious selection of blowers, jet slots and receivers. The coating compound is fed into the innermost loop, and the eddy currents created between loops by the closely spaced, oppositely directed jets transfer the coating compound to the intermediate loop and thence the outermost loop. The outer loop will have the lowest coating compound concentration and, consequently, less compound is lost to the exhaust system. The employment of recirculating loops within the coating hood has led to more uniform coatings and greater consistency in the coating operation over extended periods of time.
Another known coating hood is shown in U.S. Reissue Pat. No. 28,076, granted July 16, 1974 to B. O. Augustsson and R. S. Southwick.
Although the foregoing coating hoods perform satisfactory under most operating conditions; they may be vulnerable, however, to the effect of fumes escaping the hood.
The most obvious way of reducing the possibility of fume release from a coating hood is to increase the amount of exhaust from the hood, this has the side effect of extracting a higher percentage of circulating air from the outer air loop, thereby decreasing the efficiency of the coating process. Therefore, it is desirable to find a way of preventing fumes from entering the ambient air, while simultaneously limiting the amount of internal exhaust that is used.
Another cause of fumes escaping to the atmosphere, which must be considered, is external air currents near the hood that can enter the hood and affect flow conditions within the hood. Therefore, it is also desirable to reduce these external air currents, or at least their entrance into the hood.
Still another cause of escaping fumes results from stagnant air that is trapped between bottles as they exit the hood.
Lastly, due to the high temperatures involved with the process air, the exhaust system will have a handle a larger quantity of air than the amount of calculated ambient air intake and finish protection air.
Thus, all of these factors must be taken into consideration when constructing a coating hood to minimize escape of fumes therefrom.